Welcome to iMore! Make sure that the volume is turned up, both on the car stereo and on the phone itself. If you're still experiencing issues try removing the car's Bluetooth in Settings > Bluetooth, tapping on the "i" next to the device, then tapping on Forget This Device. Reset your device by holding the power button and home button down simultaneously until you see the Apple boot logo. After that, re-pair your phone to your car and see if the problem is fixed.